The seven seconds doesn't add up end to end, it goes into the "recent kills" pile. If you kill someone, then make another kill three seconds later, the cart will have moved for a total of ten seconds. 3 at kill level one, 4 at kill level two, then another 3 at kill level one.

You can also slow the enemy's cart to a snail's pace by standing near it. So there is still advantages to following the carts around rather than just sitting around spawn and see who kills each other faster.

The idea of kills moving the cart sounds interesting, but the idea of <i>only</i> kills moving the cart sounds like it'll just degrade into team deathmatch, and there will be no reason to do anything around what should be the map objective.

Kritzgrieg at the start of the round (and overall) could provide some massive cart movement. Just thinking about how this would play out, I think it kinda places de-emphasizes away from the actual cart. It is more of a measuring stick of how your team is performing in their kill/death ratios and not as much of how your team is working to achieve the objective. Perhaps extra bonus can be added to kills made within a certain area of your teams cart, to better give focus on the "objective"